Electrical power often starts its life pristine—a power-plant generator rotating at a constant speed to produce a smooth, constant-frequency sinusoidal alternating current signal. That signal enters the world as beautiful as a new baby, but it starts aging quickly. Strikes of lightning insert large spikes into the smooth sinusoid. Motors in Industrial equipment feed noise back into an electrical system, interfering even more with the smoothly varying voltage from the power plant. Even household appliances can cause such interference, as one can often hear through stereo equipment.
Audiophiles are not the only people who worry about noise, spikes, and other problems on their power lines. Rather, operators of other electronic equipment, such as computers, also want to have “clean” power to prevent damage to their sensitive gear. This is particularly true for operators of large commercial computer installations such as computer data centers, where thousands or hundreds of thousands of computer servers may be operating. Operators of computer centers also want constant power—no black-outs, no blips, and no sudden re-boots. As a result, such operators may install uninterruptible power supplies (UPS's) that can both condition power (e.g., to return it back to its original smooth, sinusoidal form) and provide some level of battery back-up, either to keep a system operating throughout a power failure, or to give the system enough time to shut down safely and cleanly. In a large computer data center, such a UPS may condition power as it enters the data center, and may condition many megawatts of power.
Large UPS systems suitable for large data centers are expensive to purchase and operate. Such systems can cost millions of dollars to purchase and install. Also, they are less than 100% efficient, and thus take a little power “off the top”—which may be small as a percentage basis, but can be very large for a facility that uses millions of dollars of electricity annually.